Miniature active matrix liquid crystal displays (AMLCD) require an illumination source. In systems using transmissive AMLCDs, a light source is placed behind the display. FIG. 1 shows a simple prior art approach in which a transmissive AMLCD 10 is provided with an light emitting diode (LED) backlight 20. Rays from the backlight 30 propagate through the AMLCD and are modulated to produce an image. In some AMLCDs, color is obtained by sequentially loading red, green, and blue subframes into the AMLCD, and by simultaneously sequentially illuminating red, green, and blue LEDs. Sequential illumination is accomplished by providing current sequentially through one of the desired LED leads 40. A beam shaping element 50, such as a Fresnel lens, may be used to collimate the light. Other elements, such as diffusers or filters, may also be employed. AMLCDs and illuminators of this type are available commercially from Kopin Corporation. The viewing system for such displays may comprise simple optical magnifier optics, or a multi-stage optical system characterized by intermediate image planes between the stages.
Prior art miniature reflective AMLCDs (FIG. 2) use an illuminating system that is based on a beam splitter cube 70 adjacent to the display 60. The beam splitter may comprise a polarization splitting coating 71 which serves to linearly polarize the illuminating light, and which also acts as the analyzer for the LCD. The polarizing beam splitter may alternatively be formed from polymer films. As with the transmissive AMLCD, optical elements 50 may be used to collimate, diffuse or filter the illumination.
FIG. 3 shows a more complex prior art reflective AMLCD system that includes a compact, simple magnifier added to the illuminator system for viewing a magnified image of the display 60 (U.S. Pat, No. 5,596,451). In this prior art device, a compact system is formed by using a single beam splitter 71 for illuminating and viewing the image from the AMLCD 60. Although the beam splitter 71 is used for illumination and for viewing, mirror 42, which provides the magnification, is not employed in the illumination system optical path. A lens or mirror to affect vergence of the illumination light is not needed because the light source 34 in this design is a broad area emitter.
For the case of reflective AMCLDs, projection systems have been described that employ efficient illuminators, based on lamps and collimating optics. Collimating optics provide efficient, uniform illumination of the reflective display (see, for example, U.S. Pat. No. 6,036,318). Collimation systems of this type are not employed in head-mounted displays owing to high weight and volume that results from the additional lenses and path length needed.
Collimating illumination optics are generally used in projection systems that employ high intensity lamps and projection lens systems, such as disclosed in U.S. Pat. No. 5,949,503. In some cases, such as in the patent cited, a portion of the projection optics may be used for illumination. For projection systems, this approach leads to improved illumination uniformity and improved contrast in the projected image.
This invention relates to the attainment of an improved illuminating system for reflective liquid crystal displays. The improvement is based on integrating the illumination system with the magnifying system and thus using a single set of optical elements for the two purposes of magnifying the image and illuminating the display. The invention also relates to a system for obtaining high brightness monochrome images which may be applied to reflective or transmissive liquid crystal displays.
More particularly, the invention provides a display illumination and viewing system comprising an illumination optical path and a viewing optical path. At least a portion of the illumination optical path coincides with at least a portion of the viewing optical path to form a coinciding path portion. A display comprising an active matrix liquid crystal display is located at one end of the coinciding path portion. A first lens system is located on the coinciding path portion and provides an image plane on the viewing optical path. A second lens system is located on the viewing optical path.
An illumination assembly, such as red, green, and blue LEDs, is located on the illumination optical path and off the coinciding path portion. The illumination assembly is spaced from the first lens system by a distance corresponding to the focal length of the first lens system. A reflective and transmissive element, such as a beam splitter, is located at an opposite end of the coinciding path portion to reflect light from the illumination assembly onto the coinciding path portion toward the display and to transmit light from the display along the viewing optical path. In this manner, the present invention provides a collimating illumination system for a head-mounted reflective AMLCD, offering uniform and efficient illumination, with less weight and volume than prior art systems.
In another aspect of the invention, the image display system is operable in a color mode and a monochrome mode. The display system comprises an active matrix liquid crystal display operable at a determined frame rate comprising sequential loading of red, green, and blue subframes. An illumination source comprising red, green, and blue light sources, such as LEDs, is disposed to illuminate the active matrix liquid crystal display. Illumination circuitry is provided in communication with the illumination source and includes a switch operative to switch the illumination source between the color mode to provide a color display and the monochrome mode to provide a monochrome display. In this manner, the present invention obtains increased brightness by providing the ability to switch the illuminator to a monochrome mode. In a further aspect, the invention also provides for adjusting illuminator brightness.